The present invention relates to an apparatus and method of controlling an electric power steering apparatus for assisting a steering torque by a driver (or operator) using a motor and, more particularly, to a control apparatus and method for improving straight drive stability during vehicle travel.
In a conventional control method of an electric power steering apparatus, an assisting torque always acts in a direction to assist a steering torque. A steering operation is performed such that a steering torque T.sub.D and an assisting torque T.sub.A overcome a reverse torque R, as shown in (a) and (b) in FIG. 10. In FIG. 10, reference symbol SP.sub.1 denotes a spring component of a steering shaft; and SP.sub.2, a spring component of a wheel.
As described above, since the steering shaft and the wheel have the spring components SP.sub.1 and SP.sub.2, the steering torque T.sub.D and the assisting torque T.sub.A must be transmitted to the springs so as to overcome the reverse torque R. For this purpose, even if T.sub.A =0, the steering torque T.sub.D is given by: EQU T.sub.D =R
When the steering torque T.sub.D slowly rises, as indicated by a dotted curve in FIG. 11, a straight drive direction during travel becomes ambiguous, resulting in poor straight drive stability. In order to rapidly rise the torque T.sub.D, as indicated by a solid curve in FIG. 11, the spring components can be omitted, but a limitation is imposed thereon.